drm/amdgpu: Refactor amdgpu_vram_mgr_bo_invisible_size helper

Preparation for the following fix, no functional change intended.

Cc: stable@vger.kernel.org
Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>